For example, PTL 1 discloses a method for forming a hollow structure having a shape similar to that of an assembled mandrel by layering (winding) a prepreg on the surface of the longitudinally oriented assembled mandrel, covering the assembled mandrel with a vacuum bag, and heating while vacuuming the inside of the vacuum bag, thereby heat-curing the prepreg while pressing the prepreg against the assembled mandrel.
In this forming method, as a method of easily withdrawing the assembled mandrel from the formed hollow structure after the forming is completed and the prepreg is cured, as is illustrated in FIG. 6 of this document, there is a method of providing the assembled mandrel having a structure capable of being split into three pieces including an upper mandrel, a center mandrel, and a lower mandrel in advance, and first pulling out the center mandrel and then detaching and withdrawing the upper mandrel and the lower mandrel from inner wall surfaces of the hollow structure.